1. Field of the Invention
The invention relate to reactive ion etching processes used in the fabrication of solid state devices, and in particular to the fabrication of solid state devices incorporating a layer of molybdenum silicide (MoSi.sub.2) or a layer of N+ polysilicon.
2. Description of the Prior Art
The use of wet or dry etching techniques, and in particular plasma etching and reactive ion etching, is well known in solid state technology for selectively removing materials during the fabrication of a patterned solid state device.
The areas of interest in etching materials are the control of geometric dimensions, selectivity in etching with reference to the underlying material and the photoresist, degradation of the photoresist, etch rate (which is a significant factor in throughput), and uniformity of the etch, both across a single wafer and from wafer-to-wafer. It is well documented in the prior art that wet etching techniques do not maintain the necessary linewidth control for the fine line geometries required for LSI or VLSI device fabrication. Wet etching, moreover, as a purely chemical etch, is completely isotropic. Anisotropic processes are needed for dimensional control of linewidth.
Plasma etching is a process in which a partial pressure of a gas is excited which creates a reactive gas plasma. This plasma then chemically reacts with the surface of the body to be etched.
In general, plasma etching techniques range from barrel etching to ion milling. Each of these techniques can be characterized by the degree to which the etch is either a chemical process or a physical process. Barrel etchers represent the extreme case of a purely chemical process similar to wet chemistry techniques. Like wet chemical etching, the results are isotropic etching which is unacceptable for etching aluminum because of poor dimensional control. Ion milling represents the other extreme of being a purely physical process where material is etched by means of ion bombardment only. Consequently there is little selectivity to the etching process; thus all materials are etched at approximately the same rate. In between the extremes of barrel etching and ion milling there is plasma etching, reactive ion etching (RIE), and reactive ion beam etching (RIBE).
A plasma system operating in the plasma etch mode can be defined by the reduced significance of energetic ion bombardment which results from various characteristics of the process. One such characteristic is the placement of the wafers on the anode, or grounded electrode. Secondly, this type of system generally operates at pressure above 200 microns. These characteristics reduce ion bombardment of the wafer surfaces, and therefore the process can be described as an ion assisted chemical process.
Reactive ion etching involves a significantly greater contribution of physical processes. In an RIE system the wafers are placed on the cathode which is driven by rf power. RIE is generally performed at pressures below 100 microns which increases the amount of energetic ion bombardment of wafer surfaces. In these systems the cathode will take on an average DC bias with respect to the plasma. Because of the energetic ion bombardment normally incident on the surface of the wafers, RIE systems have an inherent mechanism to achieve anisotropic etching.
Reactive ion beam etching entails an even greater extent of physical ion bombardment. In these systems the plasma is actually remote from the wafers to be etched. reactive ions are produced in a plasma and are extracted and directed at the surface of the wafers. This process differs from ion milling in that the species from which ions re produced are reactive gases rather than inert gases such as argon.
The significant advantage of plasma etching over ion milling or other mechanical etching techniques is that the different gases in the plasma generally have different etch rates and can be specifically selected as required for the process. Reactive ion beam etching (RIBE) is a combination of reactive ion etching and ion milling.
As a general rule the more chemical in nature the etch process is, the greater the selectivity between the material to be etched and the underlying material. Also, the more chemical in nature, the greater the tendency for the etch to be isotropic and the greater the amount of directionality inherent in the process.
The greatest impetus behind the switch from wet chemical etch techniques to dry plasma techniques has been the need for greater control in pattern definition. To obtain good transfer of patterns into the aluminum, undecutting must be eliminated, or in other words the etch must be anisotropic. In light of this need there has been a long standing controversy regarding the ability of the plasma etch mode to produce anisotropic etching.
Since plasma etch entails very little energetic ion bombardment, critics believe straight walls are not achievable, and thus many in the industry maintain the plasma etch mode is not the technique of choice for linewidths below 2 m.
Another factor is the nature of the material being etched. As the device feature size has been reduced, new limitations to device performance have been encountered. One significant limitation to device operation has been the relatively high resistance of polysilicon when used in scaled down interconnect structures. In recent years, efforts to surmount the resistance problem have resulted in rapid progress being made in the development of new low resistance materials for use as interconnects. The greatest advancements have been made using refractory metals and refractory metal silicides, with the most attention being given to MoSi.sub.2, TiSi.sub.2, TaSi.sub.2 and WSi.sub.2.
However, the gains that have been made in improving device performance with refractory metals or metal silicides have not been without a price. Polysilicon is successfully used as a device compatible material for many reasons, not the least of which is the fact that it can be easily patterned with relatively high selectivity over companion materials. That has not generally been true for refractory metals and their silicides. In order to achieve the precise pattern replication required for submicron geometry gates, dry processing technology must be employed. Attempts to dry process refractory metal silicide gates by either conventional plasma or reactive ion etch (RIE) techniques have often been less than satisfactory. In most devices employing refractory metal silicides, the gate metalization consists of the silicide on top of a heavily doped polysilicon layer. The etch process must therefore be anisotropic for both the silicide and the polysilicon, and have a very low relative SiO.sub.2 etch rate. Problems observed in etching this double layer structure are generally of three types. First the chemically active plasma species required to etch the refractory metal or silicide may result in an isotropic, rather than an anisotropic etch, which precludes adequate pattern dimensional control of the silicide. Second, the chemical attack of the underlying material such as N+ polysilicon may be several times faster than that of the refractory metal silicide layer, which can result in an isotropic etch condition that causes undercut voids in the structure. Finally, in instances where directional etching has been achieved for one or both of the gate structure materials, it has often been at the expense of selectivity. In such cases a failure mode may occur making it very difficult to stop the etch before penetrating the required thin oxide sublayer. Yet another severe problem is encountered when one attempts to use oxygen in a plasma-photoresist errosion. The photoresist etch is also completely isotropic leading to rapid loss of pattern stability.